After the swimming leg of the Stissing Triathlon on the morning of June 21, three volunteers from the Stissing Lake Association kayaked out from the shore to hand-pull invasive water chestnuts as part of a broader effort to combat three invasive plant species that threaten the lake’s ecosystem.
“We found far less than last year, and we also did not find them in any other locations than they have been in the past,” said Kim DeNardis, one of the volunteers and the co-president of the SLA, as well as a member of the Pine Plains Conservation Advisory Council.
Each year, the SLA — an organization made up of residents along the lake — collects water samples and sends them to the Citizens Statewide Lake Assessment Program (CSLAP), a volunteer group run by the state Department of Environmental Conservation. The 2024 report shows the lake still has safe, consistently clear water, though phosphorus levels have increased slightly. The 2025 report will most likely be published this winter.
The SLA also monitors growth of invasive plant species and takes steps to combat it. While native plants help create a balanced ecosystem and filter runoff from surrounding areas, keeping the water clean, invasive species are foreign plants without natural predators, so they tend to take over, said Zion Klos, associate professor of environmental science at Marist University. They choke out beneficial plant life and can alter oxygen levels when they become prevalent.
Invasive species most commonly enter when boats that have been on other bodies of water are launched into the lake. In addition to water chestnuts — which mostly accumulate near the boat launch — Stissing Lake has curly pondweed and Eurasian milfoil, the most common invasive species across New York state.
The milfoil is difficult to remove because of its thickness and prevalence. Some residents use water rakes or hire divers to pull them out of the lake.
“It’s everywhere,” DeNardis said. “It’s the main seaweed looking thing you’ll see just standing on the shoreline.”
Curly pondweed, another such invasive plant, has been present in Stissing Lake for at least seven years, forming dense mats that can make activities like swimming and boating in the lake difficult.
Water chestnuts are easily identified by one or two green, strawberry-like leaves on the water’s surface. They were first discovered at Stissing Lake in 2020. Volunteers have hand-pulled them each season since then, typically in June before the plants develop pods that help them spread.
Hand-pulling is more effective than cutting, which breaks the stem and allows the plants to regrow. After removal, the plants are discarded away from the water where they can dry out, Denardis said.
“You want to try to get the entire root system to come up with it,” she added. “It’s not hard to pull them out, it’s just a little bit tricky so you don’t break them.”
The CSLAP report also monitors the different nutrients present in the lake’s water, such as phosphorus and chlorophyll. Phosphorus levels correlate with chlorophyll in the water, which indicates algae presence. Both phosphorus and chlorophyll levels spiked during the pandemic then declined, but have been rising again over the past couple of years at a slower rate than during the initial surge.
Phosphorus enters the lake through stormwater runoff, lawn fertilizer, agricultural manure and nearby roadways. Native plants growing along the shoreline help filter this runoff.
“It’s not a toxin, it’s a nutrient,” Klos said. “Too much of it could lead to harmful algal blooms or an ecosystem that’s filled with way too many plants, making it hard for fish and other aquatic life to survive.”
It’s unclear whether harmful algal blooms exist in Stissing Lake because testing is expensive and difficult. Harmful algae typically appear blue-green rather than green, but experts say the difference can be hard to distinguish.
That’s why Nancy Mueller, CSLAP coordinator at the New York State Federation of Lake Associations, advised, “If you see any algae, just don’t go in the water.”
The report measures water clarity using a Secchi disk, a black-and-white disk lowered into the water until it’s no longer visible. The deeper the disk can be seen, the clearer the water. Stissing Lake’s clarity measurements remain healthy.
“That lake is very carefully monitored and we have a very good handle on any changes that happen year to year,” said Denardis, who swims in Stissing regularly. “We are fortunate to have such a beautiful lake in our area.”
The best way to prevent the introduction of invasive plants or algae is by cleaning boats and kayaks before they enter the lake.
If you notice any algae or invasive plant presence in Stissing Lake, you can report it on this invasive species database.